A safe road system is one where drivers rarely leave the road; but when they do, the vehicle and roadside are both designed to help protect vehicle occupants from death or serious harm, or at least minimize the harm. There are limits to the kinetic energy exchange that humans can tolerate, during rapid deceleration associated with a crash, before serious injury or death occurs.
A key contribution to improved road safety outcomes requires that road infrastructure and hardware be designed to take into account these errors and vulnerabilities. In the event of a crash it is desirable that the crash energy is managed to tolerable levels through either cushioning the impact or redirecting the vehicle around or over the obstruction.
In the United States for 2006, there were 17,241 single vehicle run off the road (SVROR) fatalities, representing 48 percent of the total of 42,769 fatalities (1). A review of the SVROR data shows that over 90 percent of the fatalities were a consequence of inadequate roadside treatments (1). In 73 percent of the SVROR crashes, a collision with a fixed object was the first harmful event (1). The first harmful event for a further 19 percent was an overturned vehicle (1). The data from France for 2003, shows that a similar proportion of crash deaths (21 percent) were due to collisions with ditches or embankments (2).
A review of New Zealand crash data, between 2003 and 2008 inclusive, indicates that over 90 percent of all crashes involved objects being struck on the roadside (3). Sixty percent of all objects struck were ditches (3). The higher New Zealand statistics for ditch impacts may be attributable to the generally narrower roads, and more abrupt, deeper ditch cross sections.
Ideally, if a vehicle inadvertently crosses the shoulder the driver should be able to recover safely. If the driver travels onto the roadside, the probability of a crash occurring depends upon the roadside features, such as the presence and location of fixed objects, shoulder drop-off, side slopes, ditches, and trees. If the roadside is fairly flat without objects and the soil can support the vehicle weight, then the probability of a serious crash is minimal (and indeed, in many cases the driver can fully recover and there will be no SVROR crash).
Addressing SVROR crashes presents significant challenges because of the extent of road networks, variations in traffic volumes and speed, and the random occurrence of these types of crashes. Identifying and implementing cost-effective countermeasures on road networks will continue to be an ongoing challenge.
The number of SVROR crashes can be reduced through decreasing the density of roadside hazards and their proximity to the traveled way. The severity of these crashes can be reduced through decreasing the relative obstructiveness of roadside features, such as by making roadside hardware more forgiving, or modifying side slopes to prevent rollovers.
Roadside culverts present a significant danger in the event of an accident. Roads, particularly in the countryside are often bordered by a ditch. Such ditches are designed to help drain water away from the roads and other structures, preventing water damage to the road foundation as well as preventing surface flooding. In order to maintain the integrity of the drain, intersecting roads or entrance ways and the like, have a culvert, or tube, running under and across it to allow water to continue flowing along the drain. Culverts such as this generally run parallel to the main road.
In a loss of control event where the vehicle leaves the road and enters the ditch there is a high likelihood of a head on crash into an intersecting embankment and culvert end. In such situations, the vehicles kinetic energy will generally be maintained in the direction of travel. A culvert presents a significant danger as it has been established the end will often “catch” or snag a part of the vehicle, generally the bumper, wheel or part of the under carriage, causing the vehicle to arrest or roll and increasing the chances of occupant injury or death (5).
To make such culverts safer the drainage culvert ends have been made traversable; achieved through eliminating snagging hazards. Such safety ends are especially important at road locations with a high possibility of head on crashes with parallel drainage structures that are under intersecting driveways and roads.
An intersecting road embankment can also be made safer by grading the side slope to improve traversability and safety. It is desirable to achieve a slope of about 1:4 to 1:6 (i.e. vertical to horizontal dimension ratio) or flatter (4, 5).
Untreated culvert ends under driveways or median crossings are hazardous to vehicles that have left the roadway (5). AASHTO (American Association of State Highway and Transportation Officials) and Austroads recommend the use of grated culvert safety end treatments for parallel drainage culverts with diameters greater than 900 mm (4,5). At traffic volumes above 13,000 vehicles per day a grated treatment becomes a cost effective safety treatment for culvert diameters greater than 600 mm (4, 5). It is further recommended that multiple adjacent parallel drainage culverts of any diameter have the grated safety end treatments (4). Grates are recommended for cross drainage slopes when the gap exceeds one meter. Multiple cross drainage culverts perpendicular to the road with gaps greater than 750 mm require grating (4). The slope of tube inlet and outlet structures should match the adjacent side slope (4).
It is recommended that single culverts with diameters of 600 mm or less be chamfered to match the graded side slope of the intersecting road embankment (4,5).
The problem is that it is unlikely that a small to medium size vehicle with 381 mm wheel rims or less will be able to mount a 600 mm diameter or larger culvert, given that typically the outside diameter of their wheels (i.e. rim and tire) have heights in the range of 600 mm to 660 mm. The additional obstruction of a larger unprotected culvert end or a culvert headwall will make the problem increasingly insurmountable. If a vehicle at speed were to enter and track down the ditch to the culvert it is likely that the vehicle bumper, suspension, or wheel would snag on the culvert end and the vehicle would either arrest or be launched out of control.
Vehicles with smaller wheels will have an even greater risk as the culvert end hazard is a proportionally greater obstacle. The majority of the light vehicle fleet has 381 mm or smaller wheel rims, with outside wheel diameter heights of less than 600 mm. The culvert end snagging hazard is a growing problem as the proportion of small to medium size vehicles is increasing due to economic and environmental reasons. These vehicles generally have 330 mm or 356 mm wheel rims, with an outside diameter wheel height of less than 600 mm. The effect of this trend is that increasingly smaller diameter culverts are becoming dangerous snagging hazards.
The general effect following a culvert end snag at speed will be the further loss of control due to damage to the vehicle's steering and suspension. Often this results in a severe impact on the culvert end or headwall, and/or the catastrophic flipping of the vehicle.
Current practice uses pre-cast concrete end sections with grates to reduce wheel snagging on the drainage opening. The maximum spacing of grate pipes or bars are set on 600 mm centers.
These examples are exemplified by the prior art. U.S. Pat. No. 3,587,239 illustrates a culvert construction that is beveled downwardly and outwardly and includes a grate structure of heavy construction overlying and extending the beveled area which, in the event of a vehicle impact allows the vehicle to traverse the culvert upwards, over the inclined grate.
French application number 2793820 is another example of the use of a protective grate in which the grate is produced at a sloping angle to transition the vehicle over the opening of the culvert.
A further example of an attempt to solve the problem is U.S. Pat. No. 6,769,662. This example provides a pre-cast safety end for culverts. This culvert end also has a concrete transition that guides the vehicle over the top of the culvert drain entrance to avoid the wheel engaging with the drain entrance.
All of the known culvert safety ends utilize a rigid system that is designed to withstand the impact of a vehicle and direct the vehicle over the culvert. However, they all suffer from several problems inherent to their design. In particular is the large cost and engineering difficulties associated with making and installing such a system. These structures may require heavy machinery to place the structures in place, or alternatively, large amounts of concrete and/or steel to create the appropriate deflection system. It is also a common problem for culverts and ditches to accumulate debris such as branches, dirt, gravel and other roadside detritus. This accumulation, in conjunction with a sieve like grate as given in U.S. Pat. No. 3,587,239 can cause drain blockages which require extensive and tedious maintenance. Either of these considerations may be impractical for the location for the culvert to be installed.
What is needed is a safety culvert end that assists vehicles that enter a ditch and are confronted with a culvert to safely transition out of the ditch and over the culvert without the vehicle becoming snagged on the culvert end.
It is an object of the invention to provide a culvert safety end that deforms to create a surface that transitions an errant vehicle over the culvert end or to at least provide the public with a useful alternative.